The past decade has seen the automotive industry striving to reduce vehicle weight without sacrificing overall structural integrity. As a result there has emerged a demand for relatively high strength-to-weight engineering materials, such as sheet or strip, produced from cost effective, high strength low alloy steels.
Well known in the art are HSLA steels, in the form of sheet or plate, having low carbon contents which attain high strength and toughness through the addition of Cb or V, or both, as well as additions of other microalloy strengthening elements such as Ti. The prior art is replete with disclosures teaching desirable weight percentage additions of Cb and V, singly or in various combinations, for increasing the yield strength of steel compositions. For example, U.S. Pat. No. 3,254,991 discloses Cb containing steels in plate or sheet, such steels having a yield strength of 100-175 ksi in the heat treated condition. This patent teaches that Cb is included for grain refinement and that V can be substituted for Cb in whole or in part. To obtain the same grain refinement effect when substituting V for Cb, the quantity of V added should be twice the quantity of Cb replaced. U.S. Pat. No. 3,472,707 discloses Cb and V containing structural steels that have a ferritic microstructure containing precipitated carbides and carbonitrides of Cb and/or V and contain less than 2% by volume pearlite. U.S. Pat. No. 3,897,279 discloses high strength, notch-tough, Al-killed plate steels containing Cb and V. U.S. Pat. No. 3,997,372 discloses Cb and V containing HSLA sheet steels having a yield strength above 80 ksi and indicates that Cb and V are not alternative, that is to say that Cb and V are not interchangeable and both must be present. British Pat. No. 1,123,114 discloses HSLA plate steels characterized by a fine grained ferritic microstructure containing precipitated carbide or carbonitrides of Cb and/or V.
Although the art reveals a number of patents directed to Cb and V containing high strength steel compositions, for the most part, the hot rolled products made from the HSLA steels disclosed by such patents suffer from anisotropic mechanical properties. The art does not teach Cb and V HSLA steel sheet having at least 80 ksi minimum yield strength in combination with isotropic bendability properties. Furthermore, there is no teaching of a steel sheet that attains 80 ksi minimum yield strength by controlling the Cb and V content in a specific relationship with the carbon (C) and nitrogen (N) content.
Anistropic properties in HSLA steels have been attributed in part to the sulfur content. In conventional steels the sulfur exists in the form of manganese sulfide (MnS) inclusions. At hot rolling temperatures for sheet and plate, these manganese sulfides are plastic and deform into elongated stringers in the rolling, or longitudinal, direction. The resulting shape and the distribution of sulfide stringers have a marked effect on the directional, or isotropic properties of most steel products. For example, the formability of sheet, and notch toughness of hot rolled plate, in the direction longitudinal to rolling is superior to that measured in the transverse direction.
U.S. Pat. Nos. 3,666,570 and Re. 28,790 disclose Cb or V containing HSLA sheet steels having improved formability. Zirconium or rare earth metals are added as a sulfide inclusion shape-control agent. During hot rolling these complex sulfides do not deform and thus a more isotropic steel product is produced. An alternative approach to improving the transverse proerties of steel is taught in U.S. Pat. No. 3,767,387 which discloses Cb containing high tensile strength steels having improved press shapability for making plates. The sulfur content of these steels is limited to 0.01 weight percent maximum. (Hereinafter, the amount of each element in a steel composition will be represented as a weight percent value.) A critical relationship between the total carbon and sulfur content and excellent press shapability is stated to exist such that the sum of % C plus 10(%S) should be less than 0.25. U.S. Pat. No. 4,142,922 discloses an HSLA sheet steel having yield strengths up to and in excess of 80 ksi, and containing low carbon and low manganese, very little silicon, and moderate proportions of Cb and V. The prescribed levels of C, Mn and Si afford rolled products that exhibit improved properties of toughness and formability in transverse as well as longitudinal directions without special additions of shape control agents or special processing.
S. Miyoshi et al, "Manufacture of High Strength Steel for Line Pipe in LD Converters--Manufacture of Low Sulfur Steel and REM Treated Steel," Revue De Metallurgie (April 1974), p. 395-405, disclose improving the properties of plate in a transverse direction, particularly with respect to improving the shelf energy of steel pipe. This article teaches reducing the sulfur content in control rolled steel plate for minimizing MnS inclusions elongated by hot rolling. A method is disclosed for producing extra low sulfur steel of less than 0.007 S. Miyoshi et al show the effect of the sulfur content on the transverse shelf energy of 14 mm (0.55 in.) steel plate. As the sulfur content is lowered, the shelf energy rises, particularly at a sulfur content of less than 0.010.
J. J. Bosley et al, "Steel Ladle Practices for Desulfurization and Sulfide Morphology Control at U.S. Steel," Proceedings, AIME Electric Furnace Conference, vol. 36, p. 28, 1978 mention arctic line pipe having improved transverse low temperature properties and 45-80 ksi minimum yield strength steels having good formability as two important developing applications of steels exhibiting isotropic properties. Bosley et al state that the problem of directionality in plate steel can be minimized by reducing the sulfur content in the steel product or by adding elements having greater affinity for sulfur than Mn, such as rare earth metals, to modify the plasticity of the sulfide inclusions remaining in the steel. Two methods for reducing the sulfur content and simultaneously controlling the shape of the sulfide inclusions are discussed.
There are therefore suggestions that the transverse properties of steel may be improved by reducing the sulfur content of the steel or by modifying the morphology of the sulfide inclusions. While both practices have been used in the platemaking art for a number of years, workers in the sheetmaking art have only made use of sulfide shape control and have ignored desulfurization. We believe that workers in the sheetmaking art have ignored the practices of the platemaking art for the following reasons. Workers in the platemaking art are primarily concerned about transverse toughness as determined by the Charpy test. Therefore, previous work on the desulfurization of plate steel and on the addition of sulfide shape control agents to plate steel was principally directed to improving the transverse toughness of the plate. The concerns with the mechanical properties of sheet and plate are not coextensive. That is, while bendability or formability are a mutual concern, sheared edge stretchability is of no concern in the platemaking art. Likewise, transverse toughness is of little or no concern to workers in the sheetmaking art. Therefore, no data were ever developed by workers in the sheetmaking art showing whether formability improvements could be gained by desulfurizing sheet steels. Consequently, there was no incentive to incorporate the desulfurization practices being used in the platemaking art for producing HSLA steel sheet.
Accordingly, an object of this invention is a HSLA steel sheet suitable for automotive applications which has an 80 ksi minimum yield strength that is attained by the interrelationship of particular amounts of Cb, V, C and N.
Another object of this invention is a Cb and V containing 80 ksi yield strength HSLA steel sheet possessing improved transverse bendability and improved sheared edge stretchability.
Yet another object of this invention is an 80 ksi minimum yield strength HSLA steel sheet containing Cb and V as strengthening agents and having isotropic cold bending properties and improved sheared edge stretchability.